Russian media outlets that print swear words face heavy fines under rules signed into law on Monday by the president.

The new law stipulates fines for "the use of foul language" but does not give a list of banned words, leaving journalists confused over exactly what has been prohibited.

Journalists could face up to 3,000 rouble (£62) fines, while media outlets could be made to pay up to 200,000 roubles (£4,150), if they fall foul of the regulations.

But in keeping with most Russian laws, the rules were vague, and no list of prohibited language has been issued.

Journalists were left wondering if they should replace some letters in swear words with asterisks, or whether that would also break the law. Even the Kremlin-run Russia Today headlined its story on the law: "Russia bans swear words in mass media – but which ones?"

News of the law spread as Vladimir Putin, on a visit to Germany, was accosted by Femen demonstrators. Topless activists from the Ukraine-based feminist protest group disrupted the president's visit with the words "Fuck dictator" written in English on their chests and "Fuck off, Putin" in Russian on their backs. It was unclear how Russian media would report the news.

Swearing has an unusual history in Russia: on the one hand, it is taken far more seriously than in the west and is considered inappropriate for use in polite company; on the other, it is considered an art form, with many dictionaries and manuals devoted to perfecting its use.

Daily swearing has become more popular with Russia's younger, post-Soviet generation.

"You can count the number of times that swear words appeared in the press in the past few years," said Oleg Kashin, a prominent journalist. "And when they do, it's usually a citation – for example, today's Femen protest or, several years ago, quoting a song from [the Russian rock group] Tatu."

"This new law won't change anything – it's more a symbolic thing to show the so-called imagined 'simple Russian people' that the creative classes can't live without swearing. It's a symbolic gesture to show that Putin and United Russia are protecting the people's traditions."